This invention relates to a semiconductor device (hereinafter referred to as IGBT module) wherein an insulated gate bipolar transistor and a diode are in a reverse parallel connection or are connected in parallel with each other in the opposite direction.
With development of power electronics, it is important to make an inverter apparatus having a high performance, low noise and a small size.
The insulated gate bipolar transistor (hereinafter referred to as IGBT) is a voltage drive type element with the high speed of a MOSFET and the high output characteristic of a bipolar transistor. The drive circuit of an IGBT can be made smaller as compared with the drive circuit of a bipolar transistor, which is an electric current drive type element.
The typical IGBT is usually used in a form of IGBT module. IGBT modules are semiconductor devices formed in such a manner that an IGBT and a diode, which are in a reverse parallel connection, or are connected in the opposite direction in parallel with each other, are incorporated in a package, and the electrode terminal is taken out to the outside. The IGBT modules are mainly used in inverter circuits, etc.
In order to reduce the loss in an inverter circuit, it is necessary to reduce the loss of the IGBT module. Therefore, in recent years, an improvement has been made in order to reduce the switching loss and the loss due to the ON-state voltage of the IGBT. For example, it is known that a tail electric current is the main cause of switching loss. The tail electric current is an electric current that decreases as the tail is pulled gently, after the collector electric current decreases steeply by turn-off. Reduction of the tail electric current decreases the switching loss. It is known that many cells are integrated in the same area by making the unit cells minute, so that the channel width per unit area is increased to reduce the ON-state voltage. As a loss reduction technique of the IGBT like the above, it is known that the resistivity of the n base layer of the IGBT is increased, as seen in Japanese patent Laid-open print No. 4-133355. As mentioned above, it is possible to reduce the loss of an IGBT by reducing the tail electric current and the ON-state voltage. But, when the improved IGBT, as stated above, is incorporated in IGBT modules, a voltage noise occurs.